introduce basic support for CSS expressions

This commit introduces basic support for CSS expressions such as:

- rgba()
- rgb()
- url()
- translate()
- rotate()
- etc.

For instance the following configuration file:

    {
        color: "rgba(100, 100, 100, 0.2)"
    }

Will be exported as follow in CSS:

    $color: rgba(100, 100, 100, 0.2);

and will be exported as follow in JavaScript:

    var options = {
        "color": "rgba(100, 100, 100, 0.2)"
    }